Semiconductor substrates and the layers deposited thereon (collectively referred to herein as a "wafers") absorb water vapor and other gases and impurities (e.g., hydrocarbons) when exposed to the same (e.g., when a wafer is removed from a vacuum chamber). These gases and impurities degrade film properties and therefore must be desorbed and driven off the wafer (i.e., the wafer must be degassed) before further films are deposited thereon.
One conventional degas module uses infrared radiation to heat the wafer to a desired temperature. The infrared radiation originates from an array of lamps positioned above the wafer. The wafer's temperature is measured using infrared pyrometry, a measure of the infrared radiation emitted from the wafer and, if the emissivity of the wafer is known, a measure of the wafer's absolute temperature. A major disadvantage of heating a wafer by infrared radiation and measuring its temperature by infrared pyrometry is the substantial transparency of most common substrate materials (e.g., silicon) to infrared wavelength radiation at the temperature range of interest for degassing (150-500.degree. C.). Because most substrates are transparent to infrared wavelengths, the rate at which a wafer heats is dependent on the presence of other non-transparent (i.e., energy absorbing) layers, and device patterning placed on the wafer before it enters the degas chamber. Furthermore, any layers or device patterns which may be present from previous process steps affect the wafer's emmissivity making it difficult to obtain an accurate wafer temperature measurement by infrared pyrometry.
In order to achieve heating rates independent of wafer patterning some conventional degas methods employ a heated substrate support. However, due to surface roughness of both the wafer and the substrate support, small interstitial spaces may exist between the substrate support and the wafer which effectively decreases the contact area and heat transfer rate therebetween. Particularly in vacuum environments, this decreased contact area causes heat transfer to be dominated by radiation, and thereby to be slow, (as little radiation is produced at typical degassing temperatures). These spaces interfere with and cause non-uniform heat transfer from the substrate support to the wafer. To promote more uniform heat transfer, a heat transfer gas such as argon, helium or nitrogen is often used to fill the interstitial spaces between the wafer and the substrate support. This gas has better heat transfer characteristics than the vacuum it replaces and therefore acts as a thermal medium for heat transfer from the substrate support to the wafer. Accordingly the heat transfer coefficient of such a system is dependent on the spaces between the wafer and the substrate support and on the pressure, the atomic mass and the accommodation coefficient of the heat transfer medium. Small spaces and high pressures generate the best heat transfer.
In an effort to achieve smaller spaces, more efficient heating and more uniform wafer temperatures, conventional degassing apparatuses mechanically clamp the wafer to the substrate support using a clamp ring which contacts the outer edge of the wafer's frontside (i.e., a side that faces into the chamber). The clamp ring holds the wafer against the substrate support to maintain the necessary gas pressure between the substrate support and the wafer's backside (i.e., a side that faces the substrate support); a lower pressure is therefore maintained along the wafer's frontside than the pressure along the wafer's backside. However, the opposing forces applied to the wafer by the clamp ring and by the backside gas pressure may cause the wafer to bow. For example, a 10 Torr backside pressure causes an 8 inch wafer to bow about 1 mm at the wafer's center, and causes a 12 inch wafer to bow about 5 mm at the wafer's center. This bow increases the space between the substrate support and the wafer's backside, thereby decreasing the backside pressure and reducing heat transfer. Moreover, a 10 Torr backside pressure can cause the stress in the substrate to exceed the substrate's yield strength and break the wafer.
In addition to the disadvantages described above, mechanical clamping of the wafer is undesirable because the clamp ring consumes otherwise patternable surface area and because the surface contact between the wafer and the clamp ring promotes particle generation particularly as the wafer heats and expands. Accordingly, a need exists for a degassing apparatus and method that heats wafers independent of individual wafer patterning, that reduces wafer bowing, that reduces particles generated by contact between moving parts, and that increases patternable surface area.